In my prior U.S. Pat. No. 3,452,346 granted June 24, 1969, I have disclosed a static charge detector in the form of a battery-powered sensing device, including neon tube means for grounding a signal in excess of a predetermined voltage, a gating transistor receiving a signal of voltage between predetermined limits below said predetermined voltage, and emitter follower connected to the emitter of the gating transistor, a switching transistor controlled by the emitter follower, and an oscillating transistor controlled by the switching transistor. Such devices may be conveniently packaged, and are useful as an alarm device to indicate the presence of static electricity upon objects in areas where anesthetics and other explosive gaseous media are employed. While such construction has contributed substantially to the prevention of accidents in medical operating rooms and comparable areas, the design of the disclosed devices is such that should the one transformer included in the circuitry fail, the collapse of the field created by the transformer could permit a sudden surge of voltage of a value sufficient to cause an explosion. To prevent this occurrence, I incorporated in one of the disclosed embodiments a pair of Zener diodes employed as a voltage limiting device. These are placed back-to-back across the output coil powering the transducer. The presence of the diode circuit limits such possible surge to approximately 10 volts, well below that normally required to cause an explosion where the device is operated in a hazardous atmosphere. Unfortunately, the Zener diodes themselves may also be the subject of failure, in advance of transformer failure, so that should the transformer subsequently fail, the device is completely unprotected against a subsequent voltage surge, and possible explosion. While the possibility of such disaster is not of a large order of magnitude, the consequences require that adequate protection be afforded.